1. Field of the Invention
The present invention relates to the field of plant breeding and, more specifically, to the development of broccoli plants that exhibit high yield and quality combined with tolerance to hollow/split stem disorder.
2. Description of Related Art
The goal of field crop breeding is to combine various desirable traits in a single variety/hybrid. Such desirable traits may include greater yield, resistance to insects or pests, tolerance to heat and drought, better agronomic quality, higher nutritional value, growth rate and fruit properties.
Breeding techniques take advantage of a plant's method of pollination. There are two general methods of pollination: a plant self-pollinates if pollen from one flower is transferred to the same or another flower of the same plant or plant variety. A plant cross-pollinates if pollen comes to it from a flower on a different plant.
Plants that have been self-pollinated and selected for type over many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny, a homozygous plant. A cross between two such homozygous plants of different varieties produces a uniform population of hybrid plants that are heterozygous for many gene loci. Conversely, a cross of two plants each heterozygous at a number of loci produces a population of hybrid plants that differ genetically and are not uniform. The resulting non-uniformity makes performance unpredictable.
The development of uniform varieties requires the development of homozygous inbred plants, the crossing of these inbred plants, and the evaluation of the crosses. Pedigree breeding and recurrent selection are examples of breeding methods that have been used to develop inbred plants from breeding populations. Those breeding methods combine the genetic backgrounds from two or more plants or various other broad-based sources into breeding pools from which new lines are developed by selling and selection of desired phenotypes. The new lines are evaluated to determine which of those have commercial potential.
One crop species which has been subject to such breeding programs and is of particular value is broccoli. Broccoli possesses a compact of head of florets attached by small stems, which sprout from a thick, edible stalk. Its color can range from deep sage to dark green to purplish-green, depending upon the variety.
Broccoli has become an increasingly popular crop worldwide, especially among health-conscious consumers. An average broccoli stalk may contain only 30 calories yet provide 240% of the recommended daily allowance (RDA) of vitamin C plus 10% of the RDA of vitamin A. Broccoli is also recognized as a valuable source of dietary fiber, minerals, such as calcium and magnesium, and certain glucosinolates that may have a chemoprotective effect when consumed.
Presently, the United States is the world's largest producer of broccoli. The leading broccoli-producing states are California, Arizona, Texas and Oregon. Broccoli is also grown on a large scale in Spain, northern Europe, Central America and Australia. Of the estimated 3 billion pounds of broccoli that are marketed in the United States each year, about 1 billion pounds are minimally processed and shipped fresh, ready for consumption. The fresh broccoli market in the United States consists of two main segments: (1) whole broccoli crowns and bunch broccoli and (2) broccoli florets. A broccoli crown, e.g. of the Marathon variety, is typically a single 4 or 5 inch stalk with lateral stems trimmed off. Bunch broccoli is a group of broccoli stalks, with lateral stalks trimmed off, which are held together with a rubber band or twist tie. Florets are broccoli buds that have been trimmed from a stalk.
Broccoli is a plant of the mustard family, Brassicaceae (formerly Cruciferae). It is typically classified as the Italica Cultivar Group of the species Brassica oleracea. Other cultivar groups of Brassica oleracea include cabbage, cauliflower, kale, collard greens, kohlrabi and Brussels sprouts. A popular broccoli variety sold in North America is known as Italian green, or Calabrese, named after the Italian province of Calabria where it was first grown. Other common varieties of broccoli include Pixel, a standard variety in Japan and Australia, Marathon, a world wide standard for crowns, and Heritage, a standard for Crowns and freezers in the U.S. and Mexico.
Broccoli is a diploid species with 2N=18 chromosomes and is the result of years of breeding for the suppression of flower development in the wild mustard plant. Most broccoli varieties are cross pollinated. Much of the commercial crop is grown from F1 hybrid seeds.
While breeding efforts to date have provided a number of useful broccoli lines with beneficial traits, there remains a great need in the art for new lines with further improved traits. Such plants would benefit farmers and consumers alike by improving crop yields and/or quality.